Digital communication networks can be classified into two types: synchronous and asynchronous networks. In a synchronous network, communication capacity is statically allocated to users by dividing time into fixed size frames and assigning a portion of each frame to an individual user. In an asynchronous network, information is organized into self-contained data units called packets or cells. Each data unit is transferred over a network independently without being controlled by time frames.
The advantage of a synchronous network is that each user is allocated a given communication capacity such that a certain type of quality of service, e.g., transmission bandwidth and delay bounds, can be guaranteed. However, a synchronous network is inefficient in transporting variable bit-rate traffic since communication resources can not be shared among users. Today's digital telephone network is one example of a synchronous network which supports real-time voice conversation relatively well but exhibits low efficiency when used for bursty computer data transmissions in which large amounts of data are transmitted in a burst.
Asynchronous networks, on the other hand, can achieve high network utilization by statistically multiplexing variable bit-rate traffic on a transmission link, but with a loss of quality of service guarantees for users. While present computer networks offer an example of how an asynchronous network supports computer data communication, asynchronous networks oftentimes cannot support real-time audio/video communications satisfactorily.
Asynchronous Transfer Mode or ATM technology is an emerging technology which tries to achieve the advantages of both synchronous and asynchronous networks. Specifically, ATM networks organize a user's information into 53-byte fixed-size cells which are asynchronously transferred along a pre-established path without being subject to constraints of time frames. Since transmission bandwidth is dynamically shared by all users, high network utilization can be achieved. But ATM networks inherit a common problem of asynchronous transmission: namely the lack of quality of service guarantees. It is difficult to control the amount of traffic transferred over an ATM network such that quality of service can be guaranteed while maintaining a high level of network usage. Although many traffic control schemes have been proposed to overcome this problem, no one has succeeded in making ATM capable of possessing advantages of both synchronous and asynchronous transmissions simultaneously.
Specifically, investigators have tried to solve the traffic control problem with two approaches: preventive control and reactive control. Preventive control tries to prevent a network from reaching an unacceptable level of congestion. A common approach is to control traffic flow at entry points of a network with admission control and bandwidth enforcement. For ATM switches, admission control determines whether or not to accept a new connection at the connection setup time. Bandwidth enforcement monitors each individual connection to ensure that the actual traffic flow conforms to that specified at the time of connection setup. With the use of a proper cell transmission scheduling policy at switches, preventive control can often provide users with a certain type of quality of service, ranging from a guaranteed bandwidth to delay bounds. But a major problem with preventive control is that it is difficult to achieve high network utilization for variable bit-rate traffic, e.g., traffic of widely differing bit rates. A user is usually not allowed to send more messages than specified, even when a network load condition allows this to happen. This makes ATM networks behave like synchronous networks. Also, many traffic policing and scheduling schemes are costly to implement.
Reactive control, on the other hand, admits traffic according to the current network load condition with a feedback mechanism. A network can accept traffic on a best effort basis and there is no pre-set limit on the amount of traffic that each connection can carry. Thus it is possible to achieve high degree of dynamic bandwidth sharing and a high network utilization, making reactive control suitable for bursty data traffics like those exhibited in computer networks. However, with reactive control, it is difficult to provide quality of service guarantees since the bandwidth that each connection can use depends on the actual network load condition. Like asynchronous transmission, a lack of quality of service guarantee makes reactive control incapable of supporting real-time applications requiring guaranteed bandwidth such as interactive audio/video transmissions.
In the view of the foregoing, there is a need to provide an approach which integrates synchronous transmission with asynchronous transmission in such a way that a network is capable of providing guaranteed bandwidth transmission, while simultaneously providing both dynamic bandwidth sharing and lossless transmission.
As to guaranteed bandwidth transmission, its purpose is to allow a user to establish a connection with a guaranteed bandwidth. Providing guaranteed bandwidth is necessary to make a network capable of supporting services provided by synchronous networks, such as today's circuit-switched telecommunication networks.
As to dynamic bandwidth sharing and lossless transmission, this permits a user to exceed its guaranteed bandwidth whenever resources are available. Specifically, any bandwidth not allocated or bandwidth allocated to one user but not used should be made available to all others, and it should also be guaranteed that no buffer overflows will occur due to this dynamic bandwidth sharing. Achievement of this objective allows a network to make efficient use of transmission bandwidth which is essential to support variable bit-rate traffic of today's asynchronous data communication networks.
Integration of guaranteed bandwidth transmission and dynamic bandwidth sharing brings immediate benefits to both network users and network providers if a proper billing scheme is used. One possible scheme is to calculate the cost of a connection partially based on the guaranteed bandwidth and partially based on the actual number of cells transmitted. In this way, a video conferencing user may be able to save network charges by turning off the video when the meeting gets boring. Since the connection is still there, he or she can monitor the meeting with audio and restart the video transmission immediately when some thing interesting happens. Network providers, on the other hand, can benefit from the increased network utilization.